Prepare release v6.0.0 (#403)

## heroku/ruby

### Changed

- Gem install behavior and configuration ([#402](#402))
  - Gem install path is now configured with `GEM_HOME` and `GEM_PATH` instead of `BUNDLE_PATH`.
  - Cleaning gems is now accomplished via running `bundle clean --force`. Previously it was accomplished by setting `BUNDLE_CLEAN=1`.
  - The `BUNDLE_DEPLOYMENT=1` environment variable is changed to `BUNDLE_FROZEN=1`.
  - The `BUNDLE_BIN` environment variable is no longer set.

Co-authored-by: heroku-linguist[bot] <136119646+heroku-linguist[bot]@users.noreply.github.com>

Author: heroku-linguist[bot]

buildpacks/ruby/CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [6.0.0] - 2025-03-12
+
 ### Changed
 
 - Gem install behavior and configuration ([#402](https://github.com/heroku/buildpacks-ruby/pull/402))
@@ -108,7 +110,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Initial version of Ruby buildpack in Rust (https://github.com/heroku/buildpacks-ruby/pull/93)
 - Version 2.0.0 for the first release is not a typo. There was an issue in pack where a builder with the same name and version number would reuse artifacts left on image from [prior runs which caused issues](https://github.com/buildpacks/pack/issues/1322#issuecomment-1038241038). There were prior releases of `heroku/ruby` CNB from different sources that triggered this problem. To ensure no one would encounter that issue we developed and released using a version we know has not been used before. Version 2.0 was the first major version without a prior release of `heroku/ruby` CNB from any source.
 
-[unreleased]: https://github.com/heroku/buildpacks-ruby/compare/v5.1.0...HEAD
+[unreleased]: https://github.com/heroku/buildpacks-ruby/compare/v6.0.0...HEAD
+[6.0.0]: https://github.com/heroku/buildpacks-ruby/compare/v5.1.0...v6.0.0
 [5.1.0]: https://github.com/heroku/buildpacks-ruby/compare/v5.0.1...v5.1.0
 [5.0.1]: https://github.com/heroku/buildpacks-ruby/compare/v5.0.0...v5.0.1
 [5.0.0]: https://github.com/heroku/buildpacks-ruby/compare/v4.0.2...v5.0.0

buildpacks/ruby/buildpack.toml

@@ -2,7 +2,7 @@ api = "0.10"
 
 [buildpack]
 id = "heroku/ruby"
-version = "5.1.0"
+version = "6.0.0"
 name = "Heroku Ruby"
 homepage = "https://github.com/heroku/buildpacks-ruby"
 description = "Heroku's buildpack for Ruby applications."